Reference is made to FIG. 1 showing a circuit diagram of single photon avalanche diode (SPAD) circuit 10. The circuit 10 includes a photosensitive diode 12 having an anode terminal connected to a substrate voltage node (VSUB) and a cathode terminal connected to an intermediate node 14. A quench resistor RQ has a first terminal connected to the intermediate node and a second terminal connected to a breakdown voltage node (VBD). Thus, the photosensitive diode 12 and quench resistor RQ are coupled in series between the substrate voltage node (VSUB) and the breakdown voltage node (VBD). A DC blocking capacitor C has a first plate connected to the intermediate node 14 and a second plate connected to the input of a logic circuit 16. The capacitor C functions to couple the AC signal present at the intermediate node 14 to the input of the logic circuit 16. The logic circuit 16 may, for example, comprise a CMOS logical NOT gate. The input of the logic circuit 16 may be separately biased by a circuit (not shown) at a voltage level that is compatible with CMOS circuitry. An output of the logic circuit 16 generates the SPAD circuit output signal VOUT at CMOS logic levels.
The voltage levels at the substrate voltage node (VSUB) and the breakdown voltage node (VBD) are selected so as to apply a reverse bias voltage across the photosensitive diode 12 that exceeds the breakdown voltage of the photosensitive diode 12. In response to absorption of an incident photon by the photosensitive diode 12, an electron-hole pair is generated and this triggers an ionization process that causes an avalanche of multiplication of carriers and the subsequent generation of an avalanche current.
In order to detect subsequent photons, it is necessary to quench the generated avalanche current. This quenching operation is performed by the quench resistor RQ. The quench resistor RQ is a passive circuit. In the absence of an incident photon, the quench resistor RQ has no effect on the effective reverse bias voltage across the photosensitive diode 12. However, the avalanche current that is generated in response to absorption of the incident photon flows in the quench resistor RQ and results in an exponential reduction of the amplitude of the effective reverse bias voltage across the photosensitive diode 12. The avalanche is quenched when the voltage drop across the quench resistor RQ causes the effective reverse bias voltage across the photosensitive diode 12 to fall below the breakdown voltage of the photosensitive diode 12.
The quench resistor RQ must be tolerant of high voltages (for example, voltages in excess of 14V). As a result, it is common for the quench resistor RQ to be implemented as a polysilicon resistor in an integrated circuit. A drawback of this resistor configuration is that the effective resistance of such a polysilicon resistor cannot be adjusted. There are a number of applications, however, where access to a variable resistance value for the quench resistor RQ would be advantageous.